Field of the Invention
The present invention relates to a duplexer, more specifically to a surface acoustic wave (SAW) duplexer, which operates with surface acoustic waves.
Such SAW duplexers are known, for example from Published, Non-Prosecuted German Patent Application DE 195 14 798 A1 and from Published, European Patent Application EP 0 422 637 A3.
Duplexers of this generic type are in principle frequency filters, via which different frequency bands can be passed. An exemplary embodiment of such a duplexer is a mobile telephone apparatus, in which a first frequency band, a transmission frequency band, is passed from a circuit configuration which produces it, via a transmission filter, to an antenna, and a reception frequency band which is received via the antenna is passed via a reception filter to a circuit configuration that processes it.
A fundamental problem with the duplexers of this type is the necessity to interconnect two two-port devices to form a three-port device. In the process, the transmission characteristics of the two individual filters, the transmission filter and the reception filter, should be changed as little as possible. Since the filters are normally connected in parallel on the antenna side, it is necessary for the respective other filter to have a high impedance, particularly in the passband of the filters. In the passband, the individual filters are generally matched to 50 W. The impedances of the individual filters in the passband of the respective other filter are considerably different than 50 W.
However, the impedances of the individual filters have to be transformed to the high-impedance area in order that they have a high impedance in the frequency band of the respective other filters, and represent an open circuit in the ideal case. If both individual filters are matched to 50 W, then a specific transformation is required to change the impedance outside the passband without adversely affecting the characteristics in the passband.
Until now, such transformations have been carried out by strip lines. Since SAW filters are generally capacitive outside the passband, the impedance must be shifted through more than 180.degree. until it is large enough and thus has little adverse effect on the other filter. Depending on the dielectric constant of the substrate material, this phase shift can be carried out only by a relatively long strip line. This results in the strip line occupying a large amount of space and having high losses.